Since you have taken that narrow border off, lay the qult top on your ironing board and with steam in your iron, hold it steaming just above that stretched fabric edge. It may help . Don't press it, just hold the steam above it. Let it dry. then carefully stay stitch along that edge if it has returned to where it was before. Being very careful not to stretch it at all. Support the bulk of the quilt on the table, so it won't be pulling away from where you are stay stitching. An awarding winner quilter was teaching a class on watercolor quilts. We made them in sections,and I was shocked when my sections did not line up. the teacher showed me how to stem it, and wow, it worked for me. Hope it does for you too